How to handle async with MongoDB and node.js?

Question

What is the best way to ensure that the for loop completes its processing before updating the MongoDB database in this code snippet:

var userIdArray = [] // contains 100000 userIds

User.find({'_id': {$in: userIdArray}}, 'name', function(err, result){
    var thisTime = new Date().getTime();

    var usersArray = [];
    for (var i = 0; i < result.length; i++) {
        var user = result[i];
        var userObject = {
            userId: user['_id'],
            userName: user.name,
            time: thisTime
        }
        usersArray.push(userObject);
    };

    Model.update(id,
        {$pullAll: {userIds: userIdArray}, $push: {users: {$each: usersArray}}},
        function(err, result) {
            //continue to do stuff
        }
    );
});

the async operation (Model.update) is executed after the for loop, always. Because there is nothing async inside your for loop. See: stackoverflow.com/questions/16336367/… — Salvatorelab
– Salvatorelab, Commented Mar 7, 2014 at 10:28
@TheBronx Thank you for that! So if there was another MongoDB database call inside the for loop, would that change the situation? — AmpT
– AmpT, Commented Mar 7, 2014 at 10:55
Of course Model.update will be executed after the synchronous operations in the loop, but not always after the async tasks you add inside. Node.js will "check" for completed async tasks once your synchronous code is executed (that is, once the Model.update task is queued) and will execute it's callbacks. You can read more about the Event Loop in Node.js here: blog.mixu.net/2011/02/01/understanding-the-node-js-event-loop — Salvatorelab
– Salvatorelab, Commented Mar 7, 2014 at 11:54

Mukesh Soni · Accepted Answer · 2014-03-07 10:11:18Z

2

Your for loop has all sequential and synchronous operations. It will always complete before your mongo update is triggered.

answered Mar 7, 2014 at 10:11

Mukesh Soni

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